OF ARTS AND SCIENCES. 69 



pure material, shows conclusively that the condition of the iron in the 

 Millbury vermiculite is not diiferent from that in other varieties of the 

 same family of minerals. 



Hallite. — Several years since this variety of vermiculite was sent 

 to me by Mr. John Hall, of Philadelphia, by whom it was originally 

 discovered. The examination then made showing that the mineral was 

 a new variety, if not a new species, of the vermiculite family, I gave 

 to it the name of Hallite, in recognition of the mineralogical services 

 of Mr. Hall, who not only discovered the mineral, but has carefully 

 worked the locality and observed the associations in which it is there 

 found. A preliminary notice of the mineral under this name was 

 published at the time by Professor Leeds, of Hoboken ; but the in- 

 teresting relations which the mineral bears to the subject of this paper 

 have made a further examination desirable. 



Hallite occurs in large rough six-sided prisms, with easy micaceous 

 cleavage. There are two varieties, differing markedly in color, green 

 and yellow ; and I am indebted to Mr. Hall for the following facts in 

 regard to the locality and associations of this species. Mr. Hall writes : 

 " The mineral is found at East Nottingham, in the serpentine formation 

 of south-eastern Pennsylvania, three miles south of Oxford, in Chester 

 County ; and I know of no other locality. I think the green and 

 yellow varieties are very closely related, and may possibly pass from 

 one into the other ; but I have no positive proof that they do. The 

 crystals are found in nests or pockets, and the two colors are not 

 found in the same nests. The green crystals are imbedded in a stea- 

 tite eai-th or base of the same color as the crystals, and the yellow in 

 a yellow earth ; and sometimes nests, containing the opposite varieties, 

 are only a few feet apart in seams of the serpentine rock." 



As the following analyses show, the two varieties have essentially 

 the same composition, and the only difference that could be detected 

 was in the degree of oxidation of the iron. The yellow crystals appeared 

 to be more weathered than the green, and on the last the green color 

 frequently fades out towards the centre of crystals, thus giving indica- 

 tions of a metamorphosis by which one variety may pass into the other. 



Under the microscope these scales of the mineral show a remarkable 

 appearance. Between the greenish or nearly colorless plates are seen 

 elongated scales of a yellow mineral resembling closely in color thin 

 scales of Jefferisite. They are more or less spear-shaped in form, al- 

 though usually very narrow, and lie accurately in parallel lines, which 

 cross at angles of 60° and 120^^, like the magnetic oxide of iron in the 

 Muscovite from Pennsbury, Pa., or the microscopic crystals in the 



